Does it exist? I'll be needing some service as I approach the park from the west to coordinate a meeting spot/time with another party. Should I expect any issues in terms of not being able to reach the party I am going to be trying to meet up with?

It depends on your cell phone provider. Yosemite Valley gets good coverage for ATT and Verizon due to a cell tower just northwest of Glacier Point.If your provider "roams" to ATT or Verizon (e.g., Sprint) you're OK too, if you don't mind paying for roaming.

The approaches to Yosemite will be a problem due to deep canyons and ridges. You need line-of-sight (or close to it) to the tower to get reception.

I have ATT and I just visited Yosemite and my cell reception was minimal at best. I had reception on an extended roaming network, but all I could do was text. I couldn't get or make any calls, no internet on my phone worked, no apps that used service like email, facebook, etc. And the text reception was spotty. Certain parts in my campsite would work, 5 feet away I would get no signal at all. But I would see people talking on their phones and making calls, so there is reception, it's just very very limited.